In a typical copper (Cu) pillar bump process, solder with or without nickel (Ni) below always has a larger critical dimension (CD) than the bottom of the Cu pillar, due to Cu over etching. This large top and small bottom bump profile is critical for fine pitch assembly yield, especially in bump on trace (BOT) assembly. Because a top under bump metallurgy (UBM) is closer to a neighboring joint Cu trace, there is a higher risk that the solder portion will undesirably cause a bump to trace bridge.
In addition, a conventional bump process has an inversion tin (IT) layer along the Cu pillar sidewall. This inversion tin may undesirably increase the risk of delamination due to poor adhesion with a compound material (i.e., an underfill material).
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.